Human immunodeficiency viruses type 1 and 2 (HIV-1 and HIV-2) are related retroviruses associated with the acquired immunodeficiency syndrome (AIDS) in man. A virus closely related to HIV-2, SIV.sub.mac, has been isolated from captive Macaques with AIDS. Other viruses of the same general family have been isolated from both captive and wild Old World monkeys.
The genetic structures of HIV-1 and HIV-2 are more complex than the structures of other animal retroviruses; in addition to the structural genes coding for the core and the envelope proteins, there is a tat gene, a 14Kd protein which acts in trans to increase the expression of viral genes from the viral LTR; a rev gene (19Kd) which differentially regulates the expression of virion protein; a nef gene (27Kd) which reduces viral expression; a vif gene which is essential for the infectivity of cell-free virus; and a vpr gene, the function of which is unknown. One additional gene, vpx, is present in HIV-2 and the related simian virus SIV.sub.mac, but is not present in HIV-1. The vpx protein (p16 for HIV-2 and p14 for SIV) is associated with mature virions, but its function is as yet unknown.
Most of the gene products that regulate viral expression and replication of HIV-1 are also present in HIV-2. In fact, the putative functional domains of the regulatory proteins are evolutionarily conserved. However, differences in the overall structure of the HIV-2 LTRs, which are larger than the HIV-1 LTRs, account for a variation in the responsive region to the viral transactivator gene (tat). The major structural differences between HIV-1 and HIV-2 appear to be the presence of a gene designated vpu in type 1 viruses, while a second gene designated vpx appears only in type 2 viruses. The amino acid sequences of these genes are not homologous, and their functional equivalence is not yet known.
The functions of the HIV-1/HIV-2 accessory genes have been studied by infection of T-cells in vitro, and it appears that a number of these genes are dispensable in virus replication.
As an object of the present invention, a biologically active HIV-2 clone was sought. The availability of such a replication competent HIV-2 proviral clone would permit the further study of HIV and HIV infection in man, including study of the vpx gene, the role of the "non-essential" HIV-accessory genes in viral replication, and the relevance of the structural differences between HIV-1 and HIV-2 in vivo. Additionally, the regulatory elements of the viral LTRs and their interaction with regulatory proteins in the context of a complete infectious genome could be evaluated.
The value of an infectious HIV-2 clone in developing an animal model for HIV vaccine and therapy studies should also be emphasized. Rapid progress in the development of a protective vaccine against HIV has been impaired by the lack of a suitable and cost effective animal model. Successful infection of non-human primates has been achieved only in chimpanzees and gibbons, which are scarce and therefore costly, and although infection is achieved, no pathogenicity has been observed. Thus, the development of an animal model using a human virus closely related to native HIV would be very valuable.
Finally, in view of the fact that individual isolates of HIV-1 are composed of microvariants with distinct biological properties and susceptibility to given neutralizing sera, and further because the composition of this population presumably drifts due to mutations and selection pressures which arise in response to changes in available target cells and host immunity, the availability of an infectious molecular clone would enable one to measure the genetic evolution of the HIV genome and immunological consequences of this evolution in the infected host.